thermal mass and boats

I'm considering a wood fired mass-heater for a shanty boat. The advantage of RMH is that the weight is way down low and would double as internal ballast. The trouble with traditional wood stoves, for me, is not so much an inefficient burn when damped, but more that the heat spikes when a fire is burned and everything is chilled after. Ideally, I get an even heat release throughout the day/night with one or two relatively fast burns.

I'm concerned with hot surfaces while moving around the boat, which is ALWAYS in motion if it's on the water. Also, I don't really need/want the instant heat from the barrel (well, maybe at the top for a food dryer/oven) but it seems all the options available for dealing with it add lots of weight up high where I definitely do not want it.

About one ton is the right amount of ballast for the boat I'm planning. My idea is to separate the flue at the bench into several smaller gas channels which rejoin at a bell at one end of the bench, re-separate and rejoin again at the exhaust stack, right next to and against the riser unit to use some heat to help create draft.

The goal is to get about 5k BTU/hr for 12 hours or so. I think a 6" feed/riser and a total of 9" diameter (in three 3" channels) in the bench would work. The bell at the end would be, probably, 12" cubic internal. The whole bench would be refractory cement, instead of cob, for its strength in motion.

Comments, concerns?

~ Charles

Charles, iirc, the multi chanel has been tried and doesn't work!

What about a bench bell?

When i hear you say 'shanty boat' i'm imagining a barge or maybe a scow hull, is that right? Sheltered waters, not much moving around? Towed or under power, but not sailing? I guess my main concern would be that what the boat needs to function as a boat comes first, and if a stove can fit within those parameters then carry on. It seems like it would be hard to get the weight very low in the hull as the heater is going to need a certain amount of vertical height to function and if you want it as a bench it's already above the sole.

I'd want to consider how angle of heel changes the geometry, especially if i was sailing, but even if i was just going to be rolling a bit in the slop. It might change the relative heights of inlets and outlets in the bell as the boat rolls or pitches which could do weird things to the draft. (especially if it's not symmetrical along the midline)

I'd want to build it so it held together and was absolutely secured to the hull, a few hundred pounds of stove coming adrift in a seaway could be real interesting.

If you're burning beach wood you might create pcb's although the hot fire would help. At any rate, the salt fire can also wreak havoc on metal bits pretty fast so i'd try to build it so i could replace ducting without too much hassle.

If it's down low and in the bilge a bit, watch the galvanic corrosion and careful with using dissimilar metals in contact with each other, or immersed, or connected to your hull depending on what it's made of.

If you're tied to a dock or have a stern tie or stern hook or have strong tides, then the wind will come from everywhere...if you're going to be lying to a bow anchor(s) the wind is going to hit the boat at the same angle most of the time and you,might be able to design your set up to get a draft benefit from that.

Will you have gas and or (especially) propane on board? Will you need a bilge blower and gas detectors? Don't make any wells or pockets that can trap propane settling into the bilge from a leak, you've got an ignition source right there. That might be my biggest worry.

Can you afford the amount of space this thing will take? It's going to have way way more volume than an equivalent lead ballast, and volume = more construction time and more money or a lot less living space...

I'm probably the wrong guy to listen to as I'm pretty trad. when it comes to boats...i'd probably be casting a lead ballast keel and putting in a nice cast iron woodburner. But it's an interesting idea.

Crummy that multi-channel doesn't work. Can you link to a discussion of the matter?

A bell is what I had in mind at the far end of the bench, away from the heat riser.

I'm thinking more of a pram hull; basically a scow but with about ten degrees of deadrise to give me a bilge that I can pump out if I ever need to. It will definitely sail, with an outboard kicker. Cooking will be alcohol or kerosene (actually, waste cooking oil in a kerosene stove). I've considered heating with kerosene (cooking oil) but I really want to limit my sloshy tankage to fresh water, as much as possible. I'm even considering bio-briquettes from my honey bucket and food waste as a fuel source (not for cooking.)

The draft will be about one inch per 700lbs, so a ton of ballast along the keel lowers the hull in the water quite a lot. Maybe a two ton heater would be better. Still, the boat should be stable enough even if the mass is raised a few inches from the bilge if I can figure out how to secure it properly. If I can limit the majority of the mass to 20" in height, I think it will be low enough to serve properly as internal ballast. Steel strapping bolted to the longitudinal framework members should do.

As far as encroaching on living space, etc, I would design the furniture arrangement around the heater instead of shoe-horn it in among existing furniture. Serious consideration is being given to a center-cockpit arrangement with the heater filling the space beneath. The heater would be constructed almost entirely of refractory-grade stoneware with as little metal (stainless or bronze only!) as I can possibly get away with. I could even have the inside of pipe-work glazed to smooth the flow, but that would cost quite a lot more and may create its own maintenance problems.

Or, maybe I should just be looking into a steel woodburner. I'm just trying to avoid the peak-lull effect they have on small space that can't be really well insulated.

~ Charles

Yeah, you're almost going to have to design the boat around the heater, so it had better work. Designing a sailing hull is a big undertaking just on it's own. Darn, now I'm sounding discouraging, i'm just faint-hearted about that sort of commitment. I think I would want to do the math with that center of gravity and come up with a stability curve. I'm not sure where you're sailing or if you need it to be self righting from a knockdown, but I know the outflow winds can whistle pretty good down inlets up there.

How bad will the insulation be? I don't know how you're building, if you can carvel plank that can be pretty cozy...you could cross-plank the deadrise bottom to make life easier a la george buehler...

There used to be a yahoo group 'veggie oil burners' that was pretty good. I bet you could convert a dickenson and let it tick away all winter. Even with good sized tanks you'd still be tied to town a bit though, so it would be nice to have something that ran on wood as well. It sounds like you might have an interesting story about tanks. A bit of diesel in the bilge is bad enough, can you imagine if it smelled like french fries?

Fir bark can give you a long burn into the night almost like coal...yes, smouldery and inefficient, but you have a lot of constraints to work around with a sailboat. It's not like you're heating a large space. A good sleeping bag and sleeping toque does wonders, and at least you're at sea level in salt water so your ambient temperature is somewhat moderated even up there in Alaska.

Maybe more stove folk will chime in...I bet that Ernie has thought through what you are considering, I think he's a boat guy too...

(mini thread drift if it's ok 'cause i'm curious...what for lateral resistance, lee boards? or is there an external keel under that deadrise? have you seen the Rupert Pelican? chappelle scow schooner with a junk rig on it, it might be your kind of boat...rupert pelican)

Pelican's a pretty boat and it looks pretty similar (if bigger) to what I had in mind. I was thinking a modified triloboat (triloboats.com) insulated with 2" blue construction foam board. Lee boards for lateral resistance and either a junk cat-schooner or sprit yawl, though a gaff ketch is not out of the question; I just don't know if I'm up to building a rig as complex as a gaff of this size, yet. I'm leaning heavily toward the sprit yawl because it's so simple with very few strings to bother with and is relatively easy to scandalize in all weather.

The RMH would essentially be placed where a centerboard case would otherwise go. In a boat this size (20x8 -ish), the bench wouldn't be too terribly much wider than a CB case, but would be lower, so I could use it as shelving below a countertop/table arrangement ... or whatever. The cabin furnishings would be designed around the heater and not vice-versa. Center cockpit with heater below. Head off to one side, aft stateroom with big open plan saloon and galley for'ard. Genny and batteries below saloon seating and tanks 'neath the double berth in the stateroom.

It's a full-displacement hull form, so not too much worry about overloading it, so long as I get my bow and stern lines/curves high enough so the transoms don't push/drag water when in motion. I may or may not include a cutwater at the bow, but a transom is so much easier and faster to build.

As to the idea of a multi-channel flue system in the bench with a bell at the far end, the idea was to use those large multiple pathways to reduce any goofy draft issues from being constantly in motion (even when moored.) That's also why I had thought about nine-inch total diameter in the flues with a six inch burn area - lots of choices for the gasses to flow around. Now I think on it, though, it probably would lead to backdrafting smoke because the refreshing hot gases aren't helping as much to push through the system. I'll have to have another think on that.

~ Charles

So I still haven't given up on this idea. I'm now thinking a downdraft gasification burn box with a bell on either side of a cooktop. It would be more like a tile stove than an RMH or proper masonry heater, possibly mass around 1/2 ton on a footprint of about 48"l x 30"w x 40"h. I will probably mock something up in the next couple of weeks and test for thermal retention. My biggest worry is that something this small won't retain heat very much better than a stove of mild steel.

I'm glad to see that you didn't build something with those multiple channels. The sizes you were talking about, 6" system diameter and three 3" diameter flues, would have been a major restriction. Keep in mind that a tube half the diameter has a quarter the cross section, so even three of them would give significantly less flow than the 6" flue. Add to that the fact that surface friction effectively reduces the diameter a bit, and this effect does not scale down proportionally; it would be far more significant in a 3" tube than a 6".

The bell idea is likely to work far better than the original tube(s) plan.

Charles, look for soapstone!

A smaller masonry mass may not store much more heat than a metal stove emits, but because it is less conductive than metal it will take longer for all the heat to reach the cabin. You would have to burn longer to heat up the space, but you would get some flywheel effect... maybe only a few hours of it, but still better than the almost instant on/off of a metal stove.

Kari Gunnlaugsson wrote: When i hear you say 'shanty boat' i'm imagining a barge or maybe a scow hull, is that right? Sheltered waters, not much moving around? Towed or under power, but not sailing? I guess my main concern would be that what the boat needs to function as a boat comes first, and if a stove can fit within those parameters then carry on. It seems like it would be hard to get the weight very low in the hull as the heater is going to need a certain amount of vertical height to function and if you want it as a bench it's already above the sole..........

Excellent post Kari !!!

Love the Pelican boat too !

Glenn Herbert wrote:A smaller masonry mass may not store much more heat than a metal stove emits, but because it is less conductive than metal it will take longer for all the heat to reach the cabin. You would have to burn longer to heat up the space, but you would get some flywheel effect... maybe only a few hours of it, but still better than the almost instant on/off of a metal stove.

I'm thinking much the same. The advantages of thermal mass are
1. it stays warm so the draft should stay for quite a while even after the fire goes out.
2. some residual heat bleed over a few hours after the fire goes out
3. lower immediate heat output due to absorption in the thermal mass should mean I won't get overheated by burning at clean temperatures

I suspect something like this is how the Kimberly stoves are working, but not having access to one to tear it apart, I can't confirm. Anyone have any speculation on how they achieve their claims?

I would use an insulated hot water tank somewhere deep inside the boat.
If it is full the water can't slosh.

That way the stove can be at a convenient location and the (thermal) mass at a low spot.
And when you need to get the boat out of the water or off a shoal, there is a lot of weight that can be removed easily.

If the boat needs stabilization, you could use a insulated chunk of metal (or rock) with holes for water to flow …
Dual purpose ballast weight and heat storage.